Final answer:
The correct statement is A) They always have the same chemical formula. Ionic compounds must have a balanced ratio of cations to anions to be electrically neutral, and this ratio is represented by the compound's chemical formula, like Al2O3 for aluminum oxide.
Step-by-step explanation:
The true statement about compounds is that they always have the same chemical formula. This principle is known as the law of constant composition. In every ionic compound, for instance, the total number of positive charges in the cations is equal to the total number of negative charges in the anions, ensuring electrical neutrality. This balance is represented by the compound’s chemical formula, which indicates the simplest ratio of ions that results in no net charge. For example, the formula for aluminum oxide is Al2O3, because two aluminum cations (Al³+) balance three oxide anions (O²−) to create a neutral compound.
Furthermore, conductivity characteristics depend on the state of the compound. Solutions of ionic compounds and melted ionic compounds can conduct electricity, which is not the case with the solid state. This is because in the dissolved or melted state, ions are free to move and carry electric current. However, most covalent compounds are nonelectrolytes because they do not dissociate into ions in solution, hence they usually cannot conduct electricity.